The basic principal of operation of a GM cycle refrigerator is described in U.S. Pat. No. 2,906,101 to McMahon, et al. The GM cycle has become the dominant means of producing cryogenic temperatures in small commercial refrigerators primarily because it can utilize mass produced oil-lubricated air-conditioning compressors to build reliable, long life, refrigerators at minimal cost. GM cycle refrigerators operate well at pressures and power inputs within the design limits of air-conditioning compressors, even though helium is substituted for the design refrigerants. Typically, GM refrigerators operate at a high pressure (Ph) of about 2 MPa (300 pounds per square inch absolute, psia), and a low pressure of about 0.8 MPa (117 psia). The cold expander in a GM refrigerator is typically separated from the compressor by 5 m to 20 m long gas lines. The expanders and compressors are usually mounted indoors and the compressor is usually cooled by water, most frequently water that is circulated by a water chiller unit. Some compressors are air cooled, mounted indoors and cooled by air conditioned air, or mounted outdoors and cooled by outdoor air.
Air-conditioning compressors are built in a wide range of sizes and several different designs. Means of providing additional cooling to adapt these compressors to compressing helium are different for different compressors. For example, compressors that draw approximately 200 to 600 W are typically reciprocating piston types which are cooled by adding air cooled fins to the compressor shell. Between about 800 to 4,500 W, the most common compressor is a rolling piston type with low pressure return gas flowing directly onto the compression chamber. In rolling piston compressors, oil flows into the compression chamber along with the helium and absorbs heat from the helium as it is being compressed. Most of the oil separates from the helium in the compressor shell which is at high pressure. U.S. Pat. No. 6,488,120 to Longsworth describes the cooling of helium, oil, and the compressor shell by wrapping a water cooling tube around the shell, and further wrapping a helium cooling tube and an oil cooling tube over the water tube. Cooled oil is then injected into the return helium line. In effect, the compressor serves as an oil pump. Scroll compressors that draw between 3,000 W and 15,000 W, and screw compressor that draw between 15 kW and 50 kW have been used for compressing helium, but at present the largest GM cycle refrigerators draw about 15 kW. The small reciprocating compressor has intake and exhaust valves and the rolling piston compressor compressor has a discharge valve. These valves limit the flow rate of oil that can be tolerated to flow with the oil to about 0.5% of the displacement while the scroll and screw compressors that don't have valves can pump oil that is typically about 2% of the displacement. This is sufficient to absorb about 75% of the heat from the compressor while the balance flows into the helium. Both streams flow from the compressor to be cooled external to the compressor and there is no need to remove heat from the compressor shell as is done with the smaller compressors that have valves.
Published patent application US 2007/0253854 describes a horizontal scroll compressor manufactured by Copeland Corp. which has been adapted by the same assignee as this application for compressing helium. The adaptation to flowing several times as much oil as is needed for air-conditioning refrigerants is done by having the excess oil by-pass the motor and flow directly into the scroll inlet. The Copeland compressor requires an external bulk oil separator to remove most of the oil from the helium. Heat is removed from the oil and helium in a water cooled heat exchanger, the oil is returned to the compressor and the helium passes through a second oil separator and an adsorber before flowing to the expander.
Prior art for converting this to being air cooled would replace the water cooled heat exchanger with an air cooled heat exchanger as shown in FIG. 1. This works acceptably well if the compressor is in an indoor air-conditioned environment where the air temperature is between 15° C. and 30° C. Experience has shown that heat loads of up to about 3 kW are acceptable for end users but for larger heat loads it is preferred to reject the heat to outdoor air if cooling water is not available. Designing a helium compressor to operate in an outdoor environment where temperatures can range from −30° C. to +45° C. present many challenges. The oil circulation rate is set high enough to keep the maximum discharge temperature below about 85° C. This is within an acceptable limit for the compressor but oil outgases contaminants that are adsorbed in it, principally water vapor, at a higher rate than lower temperature oil. This loads the adsorber faster and necessitates more frequent replacement of the adsorber. At low outdoor temperatures the oil becomes very viscous and makes starting the compressor difficult. This problem has been solved in the past by putting the compressor in a small shed that has adjustable louvers and a fan both of which are thermostatically controlled to keep the shed near room temperature. One or both of these features can also be incorporated in the compressor cabinet. A heater is needed to warm up the compressor before it is turned on, then the heat from the compressor keeps the shed or cabinet warm. The assignees of this application manufacture helium air-cooled compressors for operation indoors, Model CSA-71, which uses an Hitachi scroll compressor, and operation outdoors, Model CNA-61, which uses a Sanyo rolling-piston compressor. Both use prior art cooling means.
The Hitachi Corporation makes several models of scroll compressors that have been adapted to compressing helium. They draw between 5 and 9 kW. The Hitachi scroll compressors differ from the horizontal Copeland compressor in being oriented vertically and having return gas and oil flow through separate lines directly into the scroll. Helium and oil together are discharged into the shell at high pressure. Most of the oil separates from the helium and collects in the bottom of the compressor, similar to the rolling piston compressor described above. Unlike the smaller compressors, for this type of compressor, cooling the shell with a water cooling tube wrapped around it is not effective. Here, heat from the helium and oil is removed by an after-cooler that is external to the compressor shell, which is either air or water cooled. The Hitachi scroll is used to illustrate the principals of this invention because it does not need a separate bulk oil separator and the piping circuit is thus simpler.